This invention relates to self-tapping screws, and more particularly to self-tapping screws, blanks for self-tapping screws and methods for forming self-tapping screws using appropriate roll-forming dies.
It is recognized by those skilled in the art that conventional self-tapping screw products, when used to assemble thin sheet metal components, have limited reliability in service due to problems associated with a need to restrain the screw tightening torque to small relative values. This needed restraint on the tightening torque is required to minimize the potential for the anchor to strip during assembly and thereby cause the screw to spin. The anchor material is that part of the joined assembly farthest removed from the underside of the screw head. Stripping and spinning will cause a loss in assembly clamp load and subsequent deterioration of the assembly.
FIG. 1 shows a commonly encountered disadvantage of a self-tapping screw 102 that has a conventional single start thread used to join thin sheet metal components 104 and 106 into a joined assembly. The screw contains a conventional circular cross-section.
When the screw anchor material 106 (also termed a xe2x80x9cnut memberxe2x80x9d), which is the material furthest from the head of the screw, is of a width 108 equal to or less than the axial pitch 110 of the screw (defined generally herein as a xe2x80x9cthinxe2x80x9d workpiece), the leading face 112 of the coil or thread typically deflects the anchor material 106 such that the material follows the space between adjacent coils, scrolls or threads. This type of anchor material and assembly may not produce the most effective joint clamping load. In addition, inadequate thread mating contact results.
In order to overcome certain disadvantages of the single-start-thread design, when used to join thin materials, a true circular-cross-section screw having multi-lead threads that are generated around a headed blank has most recently been employed. The use of a multi-lead thread tends to better capture the anchor material, preventing the anchor material from becoming trapped between threads by providing more distributed engagement of multiple locations on the perimeter of the anchor material pilot hole.
However, even the use of multiple lead threads is not a complete solution, when taken alone. Notably, FIG. 2 shows a continuing drawback associated with conventional screws for joining thin workpieces, having either single or multi-lead threads (either circular or non-circular in cross section). The exemplary screw 200 maintains a parallel core diameter 202 as close as possible to the underside 204 of the screw head 206. This implementation can cause a reduction in the performance of the assembly. Manufacturing constraints generally create an under-filling of the screw thread crests adjacent to the underside 204 of the screw head 206, thereby creating an unspecified and non-controllable reverse thread taper 208. As such, the thread crests closer to the screw entry point are of a greater diametrical magnitude than the thread crests that are closer to the head. The reverse thread taper 208 has the disadvantage of producing a gap 210 between the internal and external mating threads of the assembly. This gap 210 has the effect of reducing mating thread contact in the essential area of the jointed structure and will result in assembly break down under lower than expected application torque.
The use of a circular cross section screw (as described above) has been heretofore preferred. While certain advantages in thread formation may be achieved using a non-circular cross-section screw, in general, it has been considered detrimental to the assembly to use a non-circular cross sectionxe2x80x94such as a commercially available roll-forming, self-tapping, multi-lobed screw. Such non-circular cross sections are viewed as lacking the necessary resistance strength to applied torque when joined into the assembly.
Accordingly it is an object of this invention to provide a self-tapping screw and an associated method for forming such a screw using a non-circular cross-section blank, thus producing a predominately multi-lobe screw with advantageous thread-forming properties, and a desirable multi-lead thread. This screw should, nevertheless, exhibit good resistance to vibrational loosening as the material relaxes between lobes notwithstanding the use of a generally non-circular cross section in the xe2x80x9cjoint clamp zonexe2x80x9d of the joined assembly.
This invention overcomes the disadvantages of the prior art by providing a headed multi-lobed blank, method of manufacturing a self-tapping screw from the blank, and resulting multi-lobed self-tapping screw for joining thin workpieces that exhibits desirable thread-forming characteristics at the conical tip and body portions, and improved resistance to vibrational loosening near the head. The resulting screw, obtained from the blank and method, incorporates a threaded tapered root portion between the screw head and body portion having a cross section that varies from nearly circular in the area adjacent to the underside of the head to a maximum out-of-round (or lobular) cross section at the intersection of the tapered root portion and the screw body. This shape, combined with a thread profile outer diameter that remains largely constant from the body portion through the tapered root portion, and a multiple helical coil (lead thread) shape, ensure more secure holding of the screw in a thin anchor materialxe2x80x94with the anchor material extruded axially forward and backward around the root portion.
The headed blank is formed, in one embodiment, by striking a generally circular or lobular-cross-section wire or rod into a die cavity of a header die. As the wire or rod is driven into the header cavity, it is plastically deformed into the desired finished blank having four sectionsxe2x80x94the head, the conical entry section, the body section and the tapered portion. The die cavity has an appropriate cross section so that the body section and conical entry point section of the finished blank is formed with a multi-lobed cross section, while the tapered portion, adjacent to the head, has an essentially circular cross section.
To form the threaded lobular screw, the finished blank is engaged by laterally moving, roll-forming dies that apply sufficient pressure to cause plastic deformation of the blank surface. The dies are maintained at an equal distance, which causes, due to the lobular cross section of the blank, an oscillating rolling rotation in the blank as one of the dies moves laterally relative to the other. This roll-forming process, in the area of the tapered root portion, creates a thread pattern that maintains an approximately constant outer diameter with respect to the body portion, but the inner (root) diameter (the valleys of each of the threads) continually tapers outwardly toward the head. In addition, the tapered root portion is provided, in this manner, with the desired maximum out-of-round near the body section and a near-circular cross-section adjacent to the underside of the head. This out-of-round variation along the tapered root portion results from the reduced forming pressure present in the larger diameter area near the head.
A screw formed in accordance with this invention provides a novel tapered root portion, adjacent to the head, having a varying cross section that creates mechanical resistance to loosening from the effects of vibration or other external forces as the material being joined by the screw relaxes between the lobes.